A camera shake (hand shake) or the like at the time of photographing by a digital camera sometimes causes a blur in an image. In order to prevent such blur, it is preferable to photograph an image at a fast shutter speed. However, since the most appropriate shutter speed is determined based on the relations among a luminance of an object, a stop value, and sensitivity of the image capturing device, for instance, in a case where an object has a low luminance, there are cases that a slow shutter speed must be set in addition to a maximum stop value in order to secure a necessary light exposure. In this case, there are measures to prevent a blur: increase the sensitivity of the image capturing device, fix the camera by a tripod, photograph with a flash, and so on.
However, when an image is photographed with an increased sensitivity of the image capturing device, the photographed image will contain conspicuous noise. Moreover, photographing with a tripod will limit places of photographing. Further, photographing with a flash cannot adapt to a long-range object where the flash lamp cannot reach.
In view of these problems, a method of substantially securing a necessary exposure amount by photographing a plurality of images at short photographing intervals at a fast shutter speed and performing a composition of the plurality of photographed images has been proposed (Japanese Patent Application Laid-Open No. 9-261526).
However, in the method of acquiring an appropriate-exposure image by the conventional plural-image compositions, a problem occurs when performing a composition of images in “slow-synchro” photographing.
The “slow-synchro” photographing is used, for instance, in a case of photographing a person in a night scene. It is designed as a measure for a case where the flash lamp reaches the person but does not reach the background, so that an intended photograph image cannot be captured. It enables to slow down the shutter speed in accordance with the lightness of the background, and in synchronization with the flash, the person and the background are captured.
FIGS. 17A to 17E are views explaining a compositions of plural-images in the slow-synchro photographing. FIGS. 17A to 17D show examples of plural pieces of images obtained in the slow-synchro photographing mode. FIG. 17A shows an image obtained with a flash, in which colors and lightness of the person are appropriate but the background is dark because of an underexposure. The images in FIGS. 17B to 17D are photographed with short exposure time without using a flash. Due to the short exposure time, all images are entirely dark. A composition of these four images produces the image in FIG. 17E. By superposing these four images, the background region can achieve appropriate lightness. However, as to the person region, despite the appropriate lightness achieved only in FIG. 17A, since image data in FIGS. 17B to 17D are superposed, appropriate colors and lightness are lost. As in this example, when the screen includes mixed regions whose appropriate exposures are different, it is necessary to perform different processing for each region.